1. Field of the Invention
The invention relates to an optical fiber with air holes extending along a longitudinal direction thereof and, in particular, to an optical fiber with the air holes sealed with a hardened resin at an end face thereof. The invention also relates to a connection structure using the optical fiber where the optical fiber is connected to another optical fiber, and an optical connector using the optical fiber.
2. Description of the Related Art
Along with the speeding up in optical communication networks and optical signal processing, higher-capacity optical fibers are desired. A spotlighted technique therefor is a photonic crystal fiber (hereinafter referred to as “PCF”) with air holes formed extending along a longitudinal direction thereof.
As shown in FIG. 7, plural air holes 73 are formed in a clad 72 surrounding a core 71 of a PCF 70. By conditioning the design (number, shape, size, configuration and the like) of the air holes 73, various properties such as ultrawideband single-mode transmission region, large effective core section area, High-Δ, and large waveguide dispersion can be realized.
As shown in FIG. 8, a holey fiber (hereinafter referred to as “HF”) 80 which is an example of total-reflection PCFs has plural (six in FIG. 8) air holes 83 formed in a clad 82 surrounding a core 81 with Ge added thereinto, whereby the effective refractive index of the clad 82 is reduced. Since the air holes 83 with a refractive index of substantially 1 are formed in the clad 82, the effective relative refractive index difference of the core 81 to the clad 82 can be increased by about 32% as compared to a general-purpose single mode fiber (hereinafter referred to as “SMF”).
As shown in FIG.9, a general-purpose SMF 90 is composed of a core 91 with a diameter small enough to satisfy single mode condition, and a clad 92 covering the core 91. Consequently, the HF 80 has the properties that the light confining effect of the core 81 can be higher than that of the general-purpose SMF 90 and the bend loss when bending the optical fiber can be reduced significantly. It is expected that, by using the properties as above, the HF is used practically as an indoor wiring optical fiber which requires necessarily being bent in wiring.
On the other hand, a conventional method for connecting optical fibers includes butt connection by mechanical splice and connector connection.
The butt connection is conducted by using a mechanical splice 100 as shown in FIG.10. The mechanical splice 100 is composed of a V-groove base plate 102 with a V-groove for supporting PCF 70 or the HF 80 and the SMF 90 where they are butted each other at respective opposed end faces and for positioning and aligning them, a covering member 103 placed on the V-groove base plate 102 to hold the PCF 70 or HF 80 and the SMF 90 inserted in the V-groove, and a clamp member 104 for clamping the V-grooved base plate 102 and the covering member 103. Between the V-groove base plate 102 and the covering member 103, wedge insertion openings 105 are formed on the side face of the mechanical splice 100. Guide holes 106 for inserting the PCF 70 or HF 80 and the SMF 90 are formed on both ends of the mechanical splice 100.
In connecting the PCF 70 or HF 80 with the SMF 90 by the mechanical splice 100, wedges (not shown) are inserted in the wedge insertion openings 105 to make a gap between the V-groove base plate 102 and the covering member 103, the PCF 70 or HF 80 and the SMF 90 are inserted through the guide hole 106 into the V-groove, the end face of the PCF 70 or HF 80 is butted to the end face of the SMF 90, and the wedges are removed to clamp the PCF 70 or HF 80 and the SMF 90 by the base plate 102 and the covering member 103.
In case of the butt connection, if an air layer is generated between the butted end faces of the PCF 70 or HF 80 and the SMF 90, Fresnel reflection on the butted end faces of the optical fibers becomes considerable. For this reason, a refractive index matching agent is previously filled in the V-groove to reduce the difference between the relative refractive indexes of the PCF 70 or HF 80 and the SMF 90. The refractive index matching agent has the same refractive index as the core of the butted PCF 70 or HF 80 and the SNF 90.
The connector connection is conducted such that optical fiber connectors with a ferrule attached to each end of optical fibers are mechanically butted each other.
As shown in FIG. 11, a ferrule 110 as a member constituting an optical fiber connector is composed of a fixing portion 112 for fixing an optical fiber bare wire with its covering removed, and a fiber holding portion 113 for holding a fiber core wire with the covering. In case of using the optical fiber connector as a single core optical connector, the ferrule 110 is cylindrical shaped. The optical fiber is fixed in the ferrule 110 with an adhesive such as a thermosetting resin, and the end face of the optical fiber and the ferrule end face 114 are polished. In general optical connectors, the end faces are polished into a spherical shape to prevent Fresnel reflection on the fiber end faces.
When connecting the PCF 70, HF 80 etc. having the air holes by the above connection methods, the following problems will arise.
In case of the butt connection by the mechanical splice 100, the refractive index matching agent filled between the PCF 70 or HFC 80 and the SMC 90 may penetrate into the air hole based on capillary phenomenon. When the refractive index matching agent with the same refractive index as that of the core penetrates into the air hole, it should be taken that a core is formed in the air hole. Therefore, light will couple with the core formed in the air hole to increase the connection loss.
In case of the connector connection, polish chips generated when polishing the end face of the optical fiber and the ferrule end face 114 may be entered in the air hole. In this case, if the connector is attached/detached repeatedly, the polish chips entered in the air hole may be exposed on the end face of the optical fiber to cause fracture or crack on the end face of the optical fiber when attaching the connector, so that the long-term reliability of the optical fiber lowers.
In order to solve these problems, a matching oil, a UV-ray setting resin or a thermosetting resin is filled in the air holes to seal the end face of the optical fiber (See JP-A-2002-236234 and JP-A-2002-323625).
The sealing of air holes at the end face of the PCF 70 or the HF 80 is conducted such that a hardening resin is filled in the air holes, and the hardening resin in the air holes is cured by ultraviolet radiation or heat. Hardening resins used generally therefor have the same refractive index as or a lower refractive index than the clad of an optical fiber.
However, the refractive index of the hardening resins exhibits temperature dependency, where the refractive index increases according as temperature lowers. As a result, a problem may arise that, at low-temperature side, the refractive index of air hole (filled with the resin) comes close to that of the core so that light is mode-coupled with the resin-filled air hole while being diverged from the core, whereby the connection loss increases.